The Romans loved astrology and did everything according to the Seven Planets, which wandered across the sky. So far as the Roman were concerned there were the Fixed Stars (all those ones in the background that never move in relation to each other), and then there were seven things visible with the naked eye that wandered around. They were the moon, Mercury, Venus, the sun, Mars, Jupiter and Saturn. These were wandering stars and in Latin each one was a stella errans, but the Greeks were still around and still speaking Greek and the Greek for wanderers is planetes. Hence planet, which just means wanderer.
Pluto may not be a planet, but the sun is.
So you’ve got the seven day week and seven planets so of course you put them together. Saturnday, Sunday, Moonday etc. That way you could have an Astrological Week, and if you were planning to start a war or fall in love you could find the Right Day To Do It. The full Latin week went like this:
Sunday = dies Solis = Sun/solar = Apollo
Monday = dies Lunae = Moon/lunar = changeable
Tuesday = dies Martis = Mars/Martian = God of War = martial as in court martial
Wednesday = dies Mercurii = Mercury = mercurial, ephemeral
Thursday = dies Iovis = Jove/Jupiter = jovial
Friday = dies Veneris = Venus goddess of love = to venerate of love something/venereal disease
Saturday = dies Saturni = Saturn = unlucky/miserable/saturnine
The Romans loved their astrological week and even tried to apply it retrospectively to the Jews, who obviously didn’t do anything on Saturday because Saturn was unlucky. You can still see the Tuesday to Friday gods in French and Spanish
Mardi Martes Mars
Mercredi Miercoles Mercury
Jeudi Jueves Jove
Vendredi Viernes Venus
And, weirdly enough, all the Roman names survive in Welsh: Dydd-sull, -llun, -marwth, -mercher, -iou, -gwener, -sadwrn.
When the Christians came along they obviously tried to Christianise things. Saturday was the sabbath and Sunday was the Lord’s Day, thus the Spanish Sabado and Domingo. But the week spread much faster than Christianity did. It arrived in Northern Europe while they were still pagan. That’s why in English (and German) the days were passed over to equivalent pagan gods.
Mars was identified with the Germanic god Tyw (who’s actually etymologically related to Zeus and Jove), hence Tuesday.
Mercury was identified with Wodin (whom you’ve heard of from the Vikings) hence Wednesday.
Thor and Jupiter were both Thunder-Gods, so Jove’s Day became Thor’s Day/Thursday in English. But in German they’re more direct and just call it Donnerstag, which literally means Thunder Day.
(That’s the same Donner that you get in the reindeer names Donner and Blitzen, which mean thunder and lightning, which is why the German army’s tactic of lightning warfare, was called the blitzkrieg, or just the blitz, which I like to think of as London being attacked by flying reindeer.)
Venus was identified with the female goddess Frigg, hence Friday.
The Northern Europeans didn’t use the planets because they weren’t into astrology, because it’s much too cloudy round these parts. In England you can detect the sun occasionally, in Wales never. The other planets might as well not exist.
But the week was just popular. It spread northward even without planets to pull it there. It is strange to think that there is no seven-day week in Homer, nor in any of the Greek dramatists; and strange to think that this arbitrary seven day cycle has been running without pause for at least two and half thousand years, probably much longer.
It is also strange to think that two thousand years ago people thought that there was a correct day of the week to be in love, as though from Saturday to Thursday you might be quite icy and indifferent and then suddenly perk up on the day of Venus. We moderns would never, ever have such a silly idea.